DNA Binding of Iron(II)-Phenanthroline Complexes: Effect of Methyl Substitution on Thermodynamic Parameters

Abstract

The influence of methyl substitution on the thermodynamic parameters for the binding of [Fe(DMP)3]2+ and [Fe(TMP)3]2+ (DMP = 4,7-dimethyl-1,10-phenanthroline, TMP = 3,4,7,8-tetramethyl- 1,10-phenanthroline) to calf thymus DNA (ct-DNA) has been studied by determining their equilibrium binding constants (Kb) at various salt concentrations and temperatures. Kb of the iron(II) complexes to ct-DNA decreases with the salt concentration in the solution, suggesting considerable electrostatic interaction in the ct-DNA binding of the iron(II) complexes. In contrast, Kb of the DNA binding increases with temperature, indicating that the DNA binding reaction of the complex is endothermic and entropically driven. The evaluation of the non-electrostatic binding constant (K0 t) based on polyelectrolyte theory has revealed that the K0 t portions of the total binding constant (Kb) are relatively large and reach 46.4% for [Fe(DMP)3]2+ at [Na+] = 0.075 M and 43.9% for [Fe(TMP)3]2+ at [Na+] = 0.100 M. The contribution of non-electrostatic binding free energy (ΔG0 t ) to total binding free energy change (ΔG0) is extremely large, i. e. > 90% for both iron(II) complexes at [Na+] = 0.05 M, suggesting that the stabilization of the DNA binding is mainly contributed from the non-electrostatic process. The effect of methyl substitution on electrostatic (ΔG0 pe) and non-electrostatic (ΔG0 t ) binding free energy changes has been systematically evaluated using the quantity of ΔΔG0 pe and ΔΔG0 t relative to that of the parent iron(II) complex, [Fe(phen)3]2+. The results indicate that the substitution of hydrogen atoms in the phen ligand by methyl groups decreases slightly the electrostatic binding free energy changes, but tremendously increases the non-electrostatic ones to yield net binding free energy changes which are more favorable for the ct-DNA binding.